Along with the recent development of mobile communication equipment, the miniaturization of a surface acoustic wave device has been requested. A surface acoustic wave element in the device is an element for converting electric information to mechanical information and then converting only necessary surface information to electric information.
A conventional surface acoustic wave device is manufactured as follows:
preparing a substrate, which has an inter digital transducer (IDT) for input-output and pads used for leading electrodes on the surface; PA1 placing and gluing the substrate to a metal or ceramic case; PA1 wire bonding the lead electrodes; and PA1 covering the case with a metal or ceramic lid. PA1 (A) method of cleaning the surfaces of the substrate and the lid, and adding voltage to interfaces between the lid and the substrate, thus bonding the lid to the substrate by anodic bonding; and PA1 (B) method of cleaning the surfaces of the substrate and the lid, treating the surfaces of the substrate and the lid to become hydrophilic, contacting the treated surfaces, and treating the contacted surfaces with heat, thus directly bonding the lid to the substrate.
More specifically, the conventional surface acoustic wave device is shown in FIGS. 22(a), 22(b) and 22(c). In these FIGS., 221 indicates a substrate; 222 is an IDT; 223 is a pad; 224 indicates a ceramic case; 225 shows an input-output terminal; 226 is a ground terminal; 227 indicates a bonding wire; 228 is an adhesive; and 229 indicates a lid.
In this conventional surface acoustic wave device, however, the wall of the case is required to be thick enough to glue the lid on sufficiently. As a result, even if the size of the substrate is small, the size of the device as a whole cannot be miniaturized. For example, a small ceramic package of a conventional surface acoustic wave device is 5 mm wide.times.5 mm long.times.1.2 mm thick. In other words, it has been difficult to miniaturize a surface acoustic wave device.
Moreover, the quality of the adhesive which is used to fix the substrate and glue the lid to the case is changed with heat, and the vapor of the adhesive is generated at high temperature such as by soldering or the like. This produces negative effects on the frequency characteristics of the wave filter, and the long-term stability of the surface acoustic wave device is lowered.
In order to solve the problems of conventional surface acoustic wave devices, Published Unexamined Japanese Patent Application Hei 1-97006, for instance, discloses a surface acoustic wave device comprising a glass container. By using a glass container instead of a ceramic or metal case, an adhesive is not required to glue a lid, thus lessening the negative effects of the adhesive on the device. However, an adhesive is still required to glue the substrate to the glass container. In other words, the conventional problems are not completely solved by this surface acoustic wave device.
In addition, Published Unexamined Japanese Utility Model Sho 63-70728 discloses a method of bonding a lid directly on a substrate. However, in this method, an adhesive is used to glue the lid to the circumference of the substrate, and the width of the circumference is required to be wide enough to sufficiently glue the lid. As a result, the miniaturization of the device is still difficult, and the vapor generated from the adhesive provides negative effects on the device.